Optical disks are spreading widely as information recording media for reproducing or recording information using laser light. Optical disks can be classified as a read-only type, a write-once type and a rewritable type. Examples of the read-only optical disks include compact disks and laser disks. Write-once or rewritable optical disks are used as information recording media. Some of these optical disks have a configuration in which an information layer is formed on one principal plane of a transparent substrate (thickness: 1.2 mm) and a protective film is formed thereon.
In recent years, a digital versatile disk (DVD) that is an optical disk with a large capacity has been commercialized. In recording/reproducing of a high-density optical disk such as a DVD, laser light with a short wavelength and an objective lens with a large numerical aperture (NA) are used. More specifically, laser light with a wavelength of 650 nm and an objective lens with a NA of 0.60 are used. The thickness of a substrate on a light-incident side of a DVD is 0.6 mm. When using one resin substrate having a thickness of 0.6 mm, the mechanical strength is low and tilt occurs, so that a DVD is formed by attaching two substrates to each other with information recording surfaces placed inside. Herein, the term “tilt” refers to an inclination between an optical axis of laser light incident upon an optical disk for recording/reproducing and a normal line to an information recording surface of the optical disk.
In order to increase further the density of information to be recorded onto an optical disk, the use of a blue purple laser light source (wavelength: about 400 nm) also is proposed. In this case, the thickness of a transparent resin layer from the surface of a substrate to a reflective layer is set to be about 0.1 mm, and a fine laser spot is formed by using a lens with a NA of about 0.85, whereby a signal is recorded/reproduced. However, a decrease in the wavelength of laser light and an increase in a NA of the objective lens decrease an acceptable value of tilt. In order to increase the acceptable value of tilt, it is effective to decrease the thickness of the resin layer on a light-incident side.
As a method for producing an optical disk in which a resin layer on a light-incident side is thin (e.g., 0.1 mm), a signal recording layer is formed on a substrate with a thickness of 1.1 mm, and a thin resin sheet is attached to the signal recording layer or the signal recording layer is coated with UV-curable resin. There also is a method in which, after the signal recording layer is formed on the thin resin sheet, the resin sheet and the thick substrate are attached to each other.
The present invention relates to an optical disk that is formed by attaching two substrates to each other. More specifically, the object of the present invention is to provide a novel method for producing a disk-shaped substrate for use in production of an optical disk, a novel method for producing an optical disk and a novel apparatus for producing an optical disk.